Aliphatic, biodegradable polyesters are usually synthesized by ring opening polymerization in the presence of metal catalysts. The most commonly used catalyst is tin(II) ethylhexanoate (Sn(oct)2), while the polymerizing process has been carried out by using melt polymerizing techniques at high temperatures above the melting temperature of the produced polyester. It brings certain disadvantages such as high energy demanding process, difficult-handling in application, coloured products due to oxidation and metal catalyst residue containing products which limits applications. In particular, it is difficult for polyglycolide to be prepared in this way because of its relatively high melting temperature.
Publication Gautier, E. et al, Synthesis and Rheology of Biodegradable Poly(glycolic acid) Prepared by Melt Ring-Opening Polymerization of Glycolide, J. Polym. Sci.: Part A: Polym. Chem., 2009, 47, 1440-1449 discloses ring opening polymerization of glycolide in melt.
Alternatively these polyesters can be prepared by solution ring opening polymerisation, but this method is not preferred for high volume industrial processes mainly due to costly and technically demanding product recovery from the polymer solute.
Publication Dechy-Cabaret, O. et al, Controlled Ring-Opening Polymerization of Lactide and Glycolide. Chem. Rev. 2004, 104, 6147-6176 discloses ring opening polymerization of lactide and glycolide in controlled manner.
Polyglycolide, or poly(glycolic acid), can also be produced by dehydration condensation polymerization with similar difficulties as described above for the ring opening polymerization of glycolide in molten state. In addition, high molecular weight is difficult to reach by condensation polymerisation.
Generally, glycolide ring opening polymerization can be achieved by cationic, metal catalyzed coordination-insertion and anionic/nucleophilic mechanisms. Only a few catalysts exists which can effectively catalyze glycolide ring opening polymerization by cationic mechanisms to high conversion and narrow molecular weight distribution. As noted, the properties and the number of applications may be further limited by metals left in the polymer when a coordination-insertion mechanism is used.
Earlier polymerization methods for preparation of polyesters and their copolymers performed by a ring opening polymerization has also lead to difficulties in collection of said polymers from reactor. In addition, making polyesters or co-polymers thereof by a melt process method has typically required pelletization of produced polyester-product before using as a plastic. And as said above, an important drawback in using melt polymerizing techniques for preparing polyesters relates to the needed high process temperature, which set tight requirements for equipments and is very energy-intensive process.
Publication Kamber, N. E. et al, Organocatalytic Ring-Opening Polymerization. Chem. Rev. 2007, 107, 5813-5840 discloses ring opening polymerization with organocatalysts.
Publication Kiesewetter, M. K. et al, Organocatalysis: Opportunities and Challenges for Polymer Synthesis. Macromolecules 2010, 43, 2093-2107 discloses organocatalysts in polymer synthesis.
Based on above, there is still a need for a more efficient, that is, low-temperature and fast method for preparing glycolide polyester by ring opening polymerization. Moreover, there is need for a method for preparing glycolide polyester by ring opening polymerization in which resulting glycolide polyester is easily collected and can be easily processed further.